Researchers from the University of Cambridge say modern humans may have spread diseases to Neanderthals in Europe and Asia after migrating out of Africa, a study published in the American Journal of Physical Anthropology reports.
“Humans migrating out of Africa would have been a significant reservoir of tropical diseases,” said co-author Dr. Charlotte Houldcroft, from Cambridge’s Division of Biological Anthropology, in a statement. “For the Neanderthal population of Eurasia, adapted to that geographical infectious disease environment, exposure to new pathogens carried out of Africa may have been catastrophic.”
Such transmission of diseases would have caused problems for Neanderthals in the same way European diseases, especially smallpox, decimated Native American populations. This is because Neanderthals — along with other archaic human species — had not developed immunity to the pathogens, making them susceptible to illness.
The diseases modern humans brought with them out of Africa included stomach ulcers, tapeworms, tuberculosis, and several types of herpes, which would have spread during interactions with Neanderthals, including interbreeding.
Previous research estimated that most major human diseases began to emerge roughly 8,000 years ago. But this recent study suggests certain diseases could be thousands of years older, Discovery News reports.
Researchers reviewed both pathogen genomes and ancient bone DNA. They found that a large number of infectious diseases have been co-evolving with humans and their ancestors for tens of thousands of years. While always present, it was not until the rise of agriculture and growth of settled communities that the pathogens were able to spread.
A stomach-ulcer-causing bacterium known as helicobacter pylori is thought to have first infected humans in Africa between 88,000 and 116,000 years ago, but did not arrive in Europe prior to 52,000 years ago. And Herpes simplex 2 — which causes genital herpes — most likely spread to humans in Africa some 1.6 million years ago.
“Hunter-gatherers lived in small foraging groups. Neanderthals lived in groups of between 15-30 members, for example,” added Houldcroft. “So disease would have broken out sporadically, but have been unable to spread very far. Once agriculture came along, these diseases had the perfect conditions to explode, but they were already around.”
The research also suggests that many diseases previously believed to be ‘zoonoses’ — illnesses transferred to humans by animals — may actually have human origins.
While anatomically modern humans migrating out Africa likely transmitted diseases to Neanderthals, genetic evidence suggests that interbreeding mostly benefited Homo sapiens by strengthening their immune systems and protecting against diseases like bacterial sepsis and encephalitis.